Artificial Light Increases Nighttime Prevalence of Predatory Fishes, Altering Community Composition on Coral Reefs

Abstract

Artificial light at night (ALAN) is an anthropogenic pollutant that is intensifying and expanding in marine environments, but experimental studies of community-level effects are generally lacking. The inshore, shallow, and clear-water locations of coral reefs and their diverse photosensitive inhabitants make these ecosystems highly susceptible to biological disturbances; at the same time, their biodiversity and accessibility make them model systems for wider insight. Here, we experimentally manipulated ALAN using underwater LED lights on a Polynesian reef system to investigate the influence on localised nighttime fish communities compared to control sites without ALAN. We collected infrared video censuses of baseline communities prior to manipulation, which we repeated following short-term (mean of three nights) and prolonged (mean of 25 nights) exposures to ALAN. Short-term ALAN exposure did not induce any significant alterations to the nighttime fish community, but prolonged ALAN exposure increased nighttime species richness. Species compositions exposed to prolonged ALAN were more dissimilar from their baseline compared to control sites. The difference between community compositions at prolonged ALAN exposure and control sites was not apparent at the family level; instead, it was observed from the composition of trait guilds. Following prolonged ALAN exposure, more diurnal and nocturnal predatory species (piscivores, invertivores, and planktivores)-particularly those that are site-attached or mobile within reefs-were present in nighttime assemblages. Our experimental findings show that coastal ALAN could cause trophic imbalances and circadian disturbances in localised nighttime reef fish communities. Given that community-wide consequences were only apparent after prolonged ALAN exposure suggests that management of the duration of artificial lighting could potentially be used to reduce impacts on marine ecosystems.

Keywords: ALAN; coral reef fish; exposure duration; field experiment; nighttime fish community; nocturnal fish; predators.

FIGURE 1

Species richness before (baseline) and after short‐term and prolonged exposure to control conditions and ALAN at (a) dusk and (b) night. (c) Temporal contrasts in the species present (β‐diversity) at night between exposure durations at control and ALAN sites. The plot is split by vertical dashed lines for each temporal contrast test: Pre‐manipulation baseline versus short‐term exposure, short‐term versus prolonged exposure and baseline versus prolonged exposure. β‐diversity index calculated using Sørensen dissimilarity, where a value of 0 indicates all species remain the same and a value of 1 indicates all species are different. In all panels, control sites are represented in blue and ALAN sites are represented in yellow; boxes denote median and interquartile range; whiskers indicate data that fall within 1.5 times the interquartile range; and contrasting letters above bars denote statistical significance. N = 16 sites.

FIGURE 2

(a) Non‐metric multidimensional scaling (NMDS) ordinations displaying the variation in the composition of trophic guilds in the nighttime fish community. Fitted labelled vectors show the most influential guilds that are driving the spread of data. Data points represent individual sites exposed to prolonged ALAN (yellow) and control conditions (blue); darker points indicate overlapping data of the same treatment while mixed colours indicate overlapping data of different treatments. The stress value obtained indicates good representations of the distribution of trophic guilds. (b) The change (from baseline to prolonged exposure) in the number of species belonging to the three most influential trophic guilds at control (blue) and ALAN (yellow) sites. Boxes denote median and interquartile range; whiskers indicate data that fall within 1.5 times the interquartile range. N = 20 sites.

FIGURE 3

(a) Non‐metric multidimensional scaling (NMDS) ordinations displaying the variation in the composition of temporal niches in the nighttime fish community. Fitted labelled vectors show the most influential niches that are driving the spread of data. Data points represent individual sites exposed to prolonged ALAN (yellow) and control conditions (blue); darker points indicate overlapping data of the same treatment while mixed colours indicate overlapping data of different treatments. The stress value obtained indicates good representations of the distribution of temporal niches. (b) The change (from baseline to prolonged exposure) in the number of species belonging to the three temporal niches at control (blue) and ALAN (yellow) sites. Boxes denote median and interquartile range; whiskers indicate data that fall within 1.5 times the interquartile range. N = 20 sites.

FIGURE 4

(a) Non‐metric multidimensional scaling (NMDS) ordinations displaying the variation in the composition of mobility types in the nighttime fish community. Fitted labelled vectors show the most influential types that are driving the spread of data. Data points represent individual sites exposed to prolonged ALAN (yellow) and control conditions (blue); darker points indicate overlapping data of the same treatment while mixed colours indicate overlapping data of different treatments. The stress value obtained indicates good representations of the distribution of mobility types. (b) The change (from baseline to prolonged exposure) in the number of species belonging to the three mobility types at control (blue) and ALAN (yellow) sites. Boxes denote median and interquartile range; whiskers indicate data that fall within 1.5 times the interquartile range. N = 20 sites.